Optically addressable spatial light modulators (SLM, SLMs, OASLM or OASLMs), as the term is used herein, are optical masks having one or more small picture elements, PELS or pixel areas that are individually and selectively switchable or writeable by the operation of one or more writing light beams or illumination. The modulator's pixel areas that have been selectively written (i.e. data has been stored therein) are then used to modulate a reading optical wavefront. In some cases, the writing wavefront and the reading wavefront may comprise a common illumination source.
Modulators of this type may be classified as intensity modulators, phase modulators, or polarization modulators.
These modulators may operate either in a transmission mode or a reflection mode. In the transmission mode of operation the writing wavefront and the reading wavefront may comprise the same wavefront, or may comprise two different wavefronts. In the reflective mode of operation the writing wavefront and the reading wavefront usually comprise two different wavefronts.
These modulators are usually two dimensional, and may comprise, for example, an X-Y coordinate system array or matrix of a plurality of small pixel areas. Modulators of this type may be arranged in a two or a three dimensional matrix of pixel rows and pixel columns.
In an OASLM, one or more write beams, for example white light or a visible laser light, programs or activates the individual pixels of the SLM so that the activated pixels subsequently operate to rotate the polarization of a read beam wavefront, for example an reading infrared laser beam. In this way the write beam programs the SLM by activating only selected photosensitive and liquid crystal pixel areas of the SLM.
U.S. Pat. No. 4,538,884 is an example of such an SLM. In the device of this patent, a pair of glass plates 1a and 2a support a pair of transparent electrodes 2a and 2b having a external source of voltage (not shown) applied thereto. A photoconductive layer 9, which can be amorphous silicon, is supported on electrode layer 2b. A plurality of aluminum reflectors 8 are incorporated into a transparent insulating layer 7 and are supported on the surface of the photoconductive layer, with the reflectors directly adjacent to the photoconductive layer. An apertured shading layer 5 of carbon or metal is carried on the transparent insulating layer, each aperture 6 facing one of the reflectors. The space intermediate transparent insulating layer 7 and transparent electrode 2a is occupied by a liquid crystal 3.
The types of known liquid crystals include nematic liquid crystals, cholestic liquid crystals, smectic liquid crystals, and chiral smectic liquid crystals, of which electroclinic smectic A and ferroelectric smectic C are two examples.
The liquid crystal material useful in the practice of the present invention is chiral smectic liquid crystal material of the distorted helix ferroelectric liquid crystal (DHFLC) type. The publication American Institute of Physics, the article entitled "Fast Responding and highly Multiplexible Distorted Helix Ferroelectric Liquid-Crystal Displays", 15 October 1989, at pages 3877 to 3882 is cited for its discussion of a liquid crystal display using a DHFLC member. It is important to note that the device of this publication is electrically addressed, rather than being optically addressed as in the present invention.
Smectic C and H ferroelectric liquid crystals are described in U.S. Pat. No. 4,367,924. This patent, however, does not describe DHFLCs, nor the unique phenomenon thereof whereby the application of an electrical field operates to change the birefringence of the DHFLC material.
A preferred light sensitive member useful in one embodiment of the present invention, but without limitation thereto, is a hydrogenated amorphous silicon (a-Si:H) photodiode layer.
The use of amorphous silicon photoconductor means (as is to be distinguished from the present invention's photodiode means) in a nematic liquid crystal SLM (as is to be distinguished from the present invention's DHFLC) is suggested in the article "Amorphous Silicon Photoconductor in a Liquid Crystal Spatial Light Modulator", by Paul R. Ashley and Jack H. Davis, APPLIED OPTICS, 15 January 1987, Vol. 26, No. 2, at pages 241-246.
The use of amorphous silicon photoconductor means and ferroelectric liquid crystal means with an intermediately located aluminum layer, to thereby form a single element light valve (i.e. not a pixelated device as in the present invention), is suggested in the article "High-speed Light Valve Using an Amorphous Silicon Photosensor and Ferroelectric Liquid Crystals", by N. Takahashi, H. Asada, M. Miyahara and S. Kurita, APPLIED PHYSICS LETTERS, Vol. 51, No. 16, 19 October 1987, at pages 1233-1235. The light valve of this article does not relate to the use of a DHFLC in a pixelated OASLM having photodiode type photosensitive member, as is described herein.
The basic configuration of an OASLM having an a-Si:H photodiode layer and a ferroelectric liquid crystal layer is taught by the article entitled "Optical and Digital Pattern Recognition", authored by G. Moddel, K. M. Johnson and M. A. Handschy, in the publication Proceedings of SPIE - The International Society for Optical Engineering, volume 754, 13-15 January 1987 at pages 207-213. This article does not describe the use of a DHFLC in a OASLM, and more particularly this article does not teach an OASLM having a DHFLC that is activated by a photodiode layer and powered by a low magnitude AC source of operating voltage, to thereby provide optical modulation of the chiral smectic liquid crystal's birefringence characteristic.
This article is of interest in that it recognizes the effect that voltages of opposite polarity have on an OASLM having an a-Si:H photodiode layer. As described in this article, in the dark (i.e. in the absence of writing illumination) the reverse biased photodiode layer passes very little current, so the reverse bias voltage is then dropped across the photodiode layer. However, when the reverse biased photodiode layer is illuminated, the photodiode layer produces a current which charges the liquid crystal layer, to thus switch pixel portions of the liquid crystal layer to the ON state. The liquid crystal layer is thereafter switched OFF by reversing the polarity of the applied bias. Under the influence of this reverse polarity, this reverse polarity voltage is dropped across the liquid crystal layer, and all pixels of the liquid crystal layer that are in an ON state are then switched OFF. The pixels that are in the OFF state remain in the OFF state.
The broad idea of using an AC source to power a liquid crystal light valve is taught by U.S. Pat. No. 3,824,002. In this connection U.S. Pat. No. 4,679,910 may also be of general interest.
While prior art OASLMs have been generally acceptable for their intended purposes, they fail to provide a low voltage OASLM, a high speed OASLM, a high resolution OASLM, analog switching of the OASLM's liquid crystal, a controllable liquid crystal birefringence characteristic as a function of the applied electric field, and the low cost OASLM that is provided by the present invention which utilizes a DHFLC. The prior art also fails to provide an OASLM with a DHFLC whose birefringence is controllable, and which provides analog or multistable switching of a DHFLC layer in an OASLM.